1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus utilizing an image forming process such as electrophotography and electrostatic recording. More particularly, the present invention relates to an image forming method and an image forming apparatus in which an image bearing member can be uniformly charged.
2. Discussion of the Background
Image forming apparatus utilizing an image forming process such as electrophotography and electrostatic recording typically include the following processes:
(1) uniformly charging the surface of a latent image bearing member such as photoreceptors (charging process);
(2) irradiating the charged latent image bearing member with light including image information to form an electrostatic latent image on the latent image bearing member (electrostatic latent image forming process);
(3) developing the electrostatic latent image with a developer including a toner to form a toner image on the latent image bearing member (developing process);
(4) transferring the toner image to a receiving material such as papers optionally via an intermediate transfer medium (transfer process); and
(5) fixing the toner image on the receiving material, resulting in output of an image (fixing process).
In the charging process, corotron chargers using corona discharging and scorotron chargers which are modified corotron chargers and which are modified so as to be able to uniformly charge latent image bearing members have been used. Corotron chargers and scorotron chargers (hereinafter referred to as charging methods utilizing aerial discharging) typically include an electroconductive wire (such as tungsten wires) and a counter electrode which is set 5 to 10 mm apart from the wire. By applying a high voltage to the wire, the molecules in the air surrounding the wire are excited and ionized, resulting in formation of charged particles. The charged particles thus formed adhere to the surface of a latent image bearing member, thereby charging the latent image bearing member.
Although the charging methods utilizing aerial discharging have an advantage such that an image bearing member can be uniformly charged, the charging methods have drawbacks such that a large amount of discharge products such as ozone and NOx are generated due to ionization of molecules in the air; and the chargers are not easy to handle.
In attempting to remedy the drawbacks, other charging methods have been developed. For example, charging rollers made of an electroconductive material such as electroconductive rubbers and resins or charging brushes made of an electroconductive material such as electroconductive fibers have been developed. By contacting such charging rollers or brushes with an image bearing member, charges are directly transported to the image bearing member (this charging device is hereinafter referred to as contact charging device). Alternatively, the chargers may be set close to an image bearing member such that charges formed by discharging at a small gap therebetween are transported to the image bearing member (this charging device is hereinafter referred to as charge injection type charging device).
The contact charging devices typically use a roller having an outermost layer made of a rubber as disclosed in published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 06-348110, 06-348112 and 06-348114.
JP-A 06-348110 discloses a charging roller having a layered structure such that an elastic layer including a pressure sensitive electroconductive rubber is formed on a metal shaft and an outermost layer is formed on the elastic layer. In the charging roller, the pressure applied to the roller is adjusted such that the image bearing member is uniformly charged so as to have a predetermined potential over a long period of time.
JP-A 06-348112 discloses a technique such that a photoreceptor is charged by a DC direct charging method so as to have a relatively low potential of from 300 to 650V. It is described therein that by using this technique, the photoreceptor can be uniformly charged and therefore good images without defective images such as streak images can be formed.
JP-A 06-348114 discloses a technique in that the electric properties of the charging roller are checked when the charging roller is located on a non-image area of the photoreceptor to be charged. If it is detected that the electric properties of the charging roller are changed, the conditions for DC charging are changed depending on the electric property changes such that the photoreceptor can be uniformly charged to have a predetermined potential.
Although these contact charging devices have an advantage such that the amount of discharge products such as ozone and NOx can be reduced, the devices have a drawback such that foreign materials remaining on the surface of the photoreceptor are transferred to the charging roller, resulting in contamination of the charging roller. When the charging roller is contaminated, the surface of the charging roller cannot be uniformly contacted with the surface of the photoreceptor, resulting in uneven charging of the photoreceptor. Alternatively, the resistance of the surface of the charging roller is changed due to the adhered foreign materials, and thereby the charging roller cannot uniformly charge the photoreceptor. When the charging roller is contaminated, the charging device including the charging roller or the process unit including the charging device has to be replaced with new one. This replacement operation is troublesome for users and increases the running costs.
In attempting to remedy the drawback of the contact charging devices, JP-A 2002-229307 discloses a short range charger in which a charging roller is set close to a photoreceptor with a small gap therebetween to uniformly charge the photoreceptor.
By using this charger, the chance of occurrence of the charging roller contamination problem can be reduced. However, foreign materials, which remain on the surface of a photoreceptor even after a cleaning process and which have a charge, fly to the surface of the charging roller, resulting in adhesion of the materials to the surface of the charging roller. In addition, when the charging roller applies a combination of a DC bias and an AC bias, charges are induced in the residual foreign materials on the surface of the photoreceptor or in the air surrounding the charging roller even when the foreign materials are not charged. The thus charged foreign materials also fly to the charging roller due to the AC bias effect. When such flying foreign materials are contacted with the charging roller, the materials are adhered thereto by means of a physical adhesion force such as van der Waals force.
The amount of the foreign materials thus adhered to the surface of a photoreceptor is much smaller than that in a case using a contact charging roller. However, when the charging roller is used for a long period of time and such foreign materials are adhered to the surface of the charging roller, the above-mentioned defective charging problem is caused.
When a short range charging device is used, a cleaning device is typically provided to clean the surface of such a short range charging roller. In this case, when a passage through which the materials collected by the cleaning device are transported to be discharged from the cleaning device is provided, the image forming apparatus is jumboized. Therefore, the collected materials are typically contained in the cleaning device. In this case, when the cleaning device is fully filled with the collected materials, the cleaning device has to be replaced with new one (i.e., the life of the cleaning device expires).
In the case of contact charging rollers, there is a possibility that the foreign materials adhered to the surface of the contact charging rollers are transferred to the surface of the photoreceptor. However, in the case of short range charging rollers, the foreign materials adhered to the surface of the contact charging rollers are hardly transferred to the surface of the photoreceptor.
Because of these reasons, a need exists for a cleaning method for removing foreign materials on a charging member without using a mechanical cleaning device such as cleaning blades.